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Old 07-03-2019, 10:14 PM   #1
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Onan 12500 - Where is the Fan Belt?

Hey Gang,

My Onan has been stopping after only running for a few minutes. Not sure if it's due to a slipping fan belt, but I thought I would start there. The generator is indicating a single flash, which means it is overheating. It otherwise runs fine and all fluids are topped off.

My question is how do I access the fan belt? The cover that pops off the side lets me access the fluids and I can see the fan, but not the belt. I popped the top cover off too by unscrewing a few bolts. Still can't see the belt. I don't see what else I can remove to see the belt other than the entire cover, which has numerous bolts and looks like it will be a 2-person job to remove. Surely I am missing something.

And if you think it could be something other than the belt, please let me know. I don't know how it could overheat after running for less than 2 minutes. Even if the belt is slipping, the fan is still spinning.

My Onan has a little over 1,000 hrs on it, BTW.
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Old 07-03-2019, 11:11 PM   #2
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I think you could feel air from the fan if you looked under the generator, there is a spot in the bottom that draws the air though to cool it.
That being said, I had a problem like that, mine would start, run and shut off too. At first it would run for a few minutes, but quickly went to starting and shutting off quickly. The problem with mine was the temperature coolant sending unit. When they go bad they short out and the ecu reads it as high temp. there are many posts about it, and depending on how you can access your gen would make a big difference on how hard the fix is.
If you can be sure it is moving air though the radiator you could disconnect the wire on the sending unit and see if it runs,,,,,,YOU BETTER BE SURE FAN IS TURNING, AIR IS MOVING! OR YOU COULD OVERHEAT THE ENGINE.
Mine would not prime for a few seconds when I hit the start button either, as the ecu thought the engine was already heated up. If your engine is shutting down in less that a minute is probably not the fan belt, I think it would take afew minutes to heat up the coolant before it would shut down.
My onan 7500 slides out the front and I can take off the top cover to see the whole top, if it didn't slide out I'd have to drop it down someway,, not fun!
Do alot of searches, it always helps.
Good luck
John G
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Old 07-03-2019, 11:16 PM   #3
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It is hard to see the belt, maybe a mirror? Is air moving? Can you reach the pulley with your hand, or, is the fan mounted to a shaft off the engine and has no belt?
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Old 07-04-2019, 05:32 AM   #4
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Quote:
Originally Posted by CJScepter View Post
I think you could feel air from the fan if you looked under the generator, there is a spot in the bottom that draws the air though to cool it.
That being said, I had a problem like that, mine would start, run and shut off too. At first it would run for a few minutes, but quickly went to starting and shutting off quickly. The problem with mine was the temperature coolant sending unit. When they go bad they short out and the ecu reads it as high temp. there are many posts about it, and depending on how you can access your gen would make a big difference on how hard the fix is.
If you can be sure it is moving air though the radiator you could disconnect the wire on the sending unit and see if it runs,,,,,,YOU BETTER BE SURE FAN IS TURNING, AIR IS MOVING! OR YOU COULD OVERHEAT THE ENGINE.
Mine would not prime for a few seconds when I hit the start button either, as the ecu thought the engine was already heated up. If your engine is shutting down in less that a minute is probably not the fan belt, I think it would take afew minutes to heat up the coolant before it would shut down.
My onan 7500 slides out the front and I can take off the top cover to see the whole top, if it didn't slide out I'd have to drop it down someway,, not fun!
Do alot of searches, it always helps.
Good luck
John G
We had the exact same problem and it turned out to be the exact same temp sensor. Our generator is on a slide so I slid it out and got underneath. Once it's slid out there is room to sit upright behind the generator. Take the cover off and you can see the fan belt very easily. Ours was pretty lose but still functional. I changed both parts and flushed the cooling system with vinegar and water. Not sure if it's the same on the larger generators. Good luck.
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Old 07-04-2019, 05:43 AM   #5
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Quote:
Originally Posted by mecdatlanta View Post
My Onan has been stopping after only running for a few minutes. Not sure if it's due to a slipping fan belt, but I thought I would start there. The generator is indicating a single flash, which means it is overheating. It otherwise runs fine and all fluids are topped off.
And if you think it could be something other than the belt, please let me know. I don't kno w how it could overheat after running for less than 2 minutes. Even if the belt is slipping, the fan is still spinning.
Did you check the coolant level by pulling the fill hose up above the highest part of the engine? It has to be full at that level, if there is an air bubble it will overheat in a couple of minutes. To gain access to the belt on my 12.5 HDKCB Spec B I have to remove the rear cover and the fan shield. Yours may have 2 belts, one for the fan and one for the water pump I think. Later they went to one belt.

If you sign on to Cummins Quickserve https://quickserve.cummins.com/info/index.html and put your generator serial number in it will let you download the latest manuals for it.
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Old 07-04-2019, 05:53 AM   #6
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I have a 12.5KW. There are 2 versions. The fan is a squirrel-cage type and is only visible from the rear. Your 2002 is most likely the early version and has 2 belts with an idler. The bad news is that with 1000 hours you probably need new belts. The slightly better news is you probably need a new temp sensor which is accessible from the top. Assuming you can remove the top cover and the rear cover, the belt change , including removing covers and cleaning all parts, takes 8 hours. You will need to throw your clothes away ... dirtiest job I've done in years .... the deteriorating belts leave a rubbery mess on everything. Yup ... 8 hrs ... yup, over $1000 for a belt change.
From my notes:
Spec A has an idler assembly for the fan.

Belt sizes used are a tight fit ... the alternator was removed/replaced to allow the belt to be installed. The fan belt was "rolled" onto the pulleys.

Alternator belt = 40-41" ID X 1/2"
Fan belt = 30-30 1/2" ID X 1/2" (NAPA A30 was used).

Belts were badly frayed at 1000 hours (On a car this would probably be 40-50k miles).

This is a difficult job and takes 8-10 hours (Including cleaning of fan and baffle).
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Old 07-04-2019, 05:57 AM   #7
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From Onan manual: Fan belt not shown in this drawing - but shows location of fan, and states that fan must be removed to replace the belt.
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Old 07-04-2019, 08:26 AM   #8
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Thanks everyone. And happy Independence Day!

It sure sounds like a sensor issue. I'll explore that avenue first. I've only owned my coach for a yr and the previous owner was meticulous about maintenance. He said he had just gotten it serviced. Whether that includes belt replacements or not, I have no idea. Unfortunately, he was going senile and when he gave me the final tour of the bus he told me to throw water down the backside of the toilet (where the electronics are) to "prime it". Fortunately, I didn't follow all of the advice he gave, including that one. And I don't think he would remember if/when the belts were replaced if I could ask him the question today.

I do think there is plenty of air flow. I went under the generator and saw the intake, but didn't think to put my hand next to it to see if there is air flow. Unfortunately, it only runs for about a min now, so I would have to be quick, or have somebody start it while I am under it.

I included a couple of pics of the two panels that were easily removed. It looks like to even look at the belt(s) I would have to do a lot of unbolting to remove the entire cover. Let's hope that it's the temperature coolant sending unit as John G suggests. It certainly sounds like there is a good chance it is.

Thanks for all the links and suggestions. I have a bit of work to do, but I at least have a better idea of what it could be.
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Old 07-04-2019, 08:41 AM   #9
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I meant to include pics of what I see behind the two removed panels. Including those now.
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Old 07-04-2019, 09:51 AM   #10
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Water Temp Sender
https://www.ebay.com/itm/Genuine-Ona...UAAOSwaNBUbjoe
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Old 07-04-2019, 10:36 AM   #11
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Can't comment on the temp sensor, but the belts are at the end of the generator that is buried in the coach. Getting at them is very difficult. I changed mine in a field at a dog show and later when I had the stator repaired at a Cummins shop the mechanic was astonished I could do it without pulling the whole unit, which is how they do it, and even then they dismount the engine from the bottom of the frame and slide it forward.

It can be done, but it requires removing a lot of nuts and bolts to move things around and get the belts in past the squirrel cage, working on the ground under it and on top.

BTW, one belt turns the fan, the other the water pump. It can overheat in a minute without the water pump, and the fan belt is almost guaranteed to take out the other belt when it fails.
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Old 07-04-2019, 11:44 AM   #12
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Quote:
Originally Posted by bruceisla View Post
I have a 12.5KW. There are 2 versions. The fan is a squirrel-cage type and is only visible from the rear. Your 2002 is most likely the early version and has 2 belts with an idler. The bad news is that with 1000 hours you probably need new belts. The slightly better news is you probably need a new temp sensor which is accessible from the top. Assuming you can remove the top cover and the rear cover, the belt change , including removing covers and cleaning all parts, takes 8 hours. You will need to throw your clothes away ... dirtiest job I've done in years .... the deteriorating belts leave a rubbery mess on everything. Yup ... 8 hrs ... yup, over $1000 for a belt change.
From my notes:
Spec A has an idler assembly for the fan.

Belt sizes used are a tight fit ... the alternator was removed/replaced to allow the belt to be installed. The fan belt was "rolled" onto the pulleys.

Alternator belt = 40-41" ID X 1/2"
Fan belt = 30-30 1/2" ID X 1/2" (NAPA A30 was used).

Belts were badly frayed at 1000 hours (On a car this would probably be 40-50k miles).

This is a difficult job and takes 8-10 hours (Including cleaning of fan and baffle).

Maybe the larger units are more difficult but from what I remember it was more difficult taking off the panels than it was changing the belt and the temp sensor. It was not even that dirty because I could sit upright where the generator usually rides when the RV is in motion. It was 3 to 4 hours tops and I flushed the engine and changed the thermostat.
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Old 07-04-2019, 01:10 PM   #13
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Originally Posted by BigLar368 View Post
Maybe the larger units are more difficult but from what I remember it was more difficult taking off the panels than it was changing the belt and the temp sensor. It was not even that dirty because I could sit upright where the generator usually rides when the RV is in motion. It was 3 to 4 hours tops and I flushed the engine and changed the thermostat.

It depends on your RV construction. On our Newmar, the Onan is on a slide with a hydraulic ram. There is a crossmember welded in place that prevents the cover from moving straight back. I was able to rotate the cover (all bolts removed) toward the front and slowly reposition the Onan slide until the cover would come off (to the front). Some folks have cut the crossmember and rewelded. If I remember correctly, the driver side, top, and back covers need to be removed. Be careful with the bolts ... there are different lengths.
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Old 07-04-2019, 01:52 PM   #14
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Originally Posted by bruceisla View Post

Belt sizes used are a tight fit ... the alternator was removed/replaced to allow the belt to be installed. The fan belt was "rolled" onto the pulleys.
I cringe everytime I see millwrights at work install a belt like that .

It practically guaranties a short life for that belt!
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